In Prior Art for cooking of chemical cellulose pulp with continuous digesters it has been a well known practice to use a vertical digester vessel, with an established down-flow process developed in the vessel from the upper inlet end and down to the lower outlet end.
During the first 3 decades of implementation of such continuous digesters was a long countercurrent washing zone established in bottom of the digester, most often over some 40-50% of the total height of the digester. This type of cooking process is often recited as “conventional cooking”.
However, as higher production capacity came into demand this wash zone was reduced to only the final 5-10% of the total pulp retention time of the digester, while using more of the retention time in the digester for the actual cooking or delignification process. “Modified continuous cooking” was thus launched; having many variants such as ITC-cooking (Iso Thermal Cooking), EAPC-cooking (Enhanced Alkali Profile Cooking) etc.
Many of the old digesters, having been designed for “conventional cooking” or “Modified continuous cooking” also tried to increase production capacity, and this resulted in that the digester most often became overloaded, and the originally intended wash zone lost some of its function as a wash effect. Then some of the washing had to be done in subsequent wash equipment.
Attempts have been made to design and improve the bottom wash of the digester. As early as 1969 was U.S. Pat. No. 3,475,271 issued, where the wash zone was more or less inverted in relation to common wash zones. Common wash zones had extraction screens in the wall of the digester, while adding wash liquid to bottom via nozzle below the extraction screens or even integrated with the bottom scraper. In U.S. Pat. No. 3,475,271 the system was inverted, such that wash liquid was added via “distribution screens” in the wall of the digester, and having a tubular extraction screen co-rotating with the bottom scraper. However, this system was no success as it included a complicated and expensive tubular body as long as the digester itself and co-rotating with the bottom scraper. The tubular body also had to have a diameter in the range of ⅕ to ⅓ of the diameter of the digester vessel, which meant that a large volume of the digester was not used for the important cooking process, all in order for establishment of sufficient extraction screen area for being able to withdraw the volumes of spent liquor.
Similar tubular central screen body was also used in some smaller pin chip digesters installed in the early 70-ies. In a typical installation in a pin chip digester with total height of about 22 meter was the height of the tubular screen body about 7.5 meter, i.e. roughly ⅓ of the total digester height. The diameter of the screen body was about 1.5 meter in the vessel having a diameter of about 3.7 meter. This meant that a large volume of the total digester volume was not available for the cooking process and hence a low production capacity per volume unit of the digester.
The concept with central screen bodies was also shown in WO2005/116327 and WO2005/116328, but in these embodiments was the central screen body integrated with the stationary central pipe. As the screen body was implemented in the central pipe, having a relatively small diameter, a limited withdrawal capacity could be obtained.
The above mentioned disadvantages with central tubular screen bodies included spacious screen bodies reducing the total volume of the digester, and had only a reduced rubbing action from the descending chip column on the screen surface for maintaining this screen surface free from blocking objects (i.e. chips in differing state of delignification)
The principle object of the invention is to obtain an improved withdrawal capacity in the bottom of the digester, while still not being spacious and reducing the available volume in the digester used for the cooking process.
A specific objective is to enable an improved wash zone in the bottom of the digester and especially for those digesters that are operating in an overloaded state such that the original wash effect in the bottom of the digester has disappeared. Thus suitable for an up-grade in those overloaded digesters.
Yet another specific objective is to be able to maintain a high withdrawal capacity in a relatively small screen area, by increasing the rubbing action from the descending pulp column keeping the draining apertures of the screen clean, which is made possible by exposing the screen area for an increased vertical downward thrust from the descending pulp column.
The invention can advantageously be used when cooking hard wood and softwood chips, bagasse and other annual plants.
The characteristics of the invention are defined by the independent claims, and optional embodiments are defined in dependent claims in order of dependency of preceding claims. The invention is also disclosed in a preferred embodiment, but any specific feature of this embodiment could as such be included in the invention optionally, if not specifically defined as a necessary feature for the argued effect.